Episode 13: What Are the Metaphysical Implications of Quantum Physics?

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On Werner Heisenberg’s “Physics and Philosophy” (1958), and talking about it with an actual former particle physicist, Dylan Casey.

What weird stuff about reality does quantum physics imply? Is Heisenberg (of the Uncertainty Principle fame) right that we need to reject “metaphysical realism” based on this very well established scientific framework? The discussion ranges over the uncertainty principle, relativity, wave/particle duality, Pre-Socratic metaphysics, why Kant is wrong about space, and lots of very weird things.

Read the text online or purchase it.

Plus, we spend far too much time talking about an article by Thomas Nagel about intelligent design; you can read that here. And the blog post by Brian Leiter that got us talking about it is here.

End song: “Neutrino of Love,” written and sung by Dylan Casey, with backing and production by Mark back in 1997 or so (remixed and cleaned up just now). A different version appears on his Neutrino Sessions album.


  1. Dan B

    January 18, 2010

    I enjoyed the episode. Quantum mechanics is very bizarre and I get the feeling after studying it that although we have learned many interesting things, we are still at a very primitive stage in our understanding of the world. One of the things I find interesting is the meaning of observation. Observations occur after and during all sorts of processing by the brain, so it seems natural that observing some system involves all sorts of interaction with the information about it, which would affect the status of the information that is immediately available in the brain. There is no way of knowing how much of what we observe actually relates to things outside of our heads. It seems lot of the focus comes back to how the brain works to process information and other philosophical problems about the mind. Our whole observation of the experimental setup in the two slit experiment is taking place within our brains, including all the measurements. I wonder if the problem has something to do with the status of information transferring from one place to another or one observer to another or something to do with the way our brains process information. I think you mentioned something along those lines but dismissed it without giving any details about the views. One of the hopes of science is that it can lead to some better answers and progress on some of these issues, but it also seems to open new doors that further confound us. You presented a choice between the copenhagen interpretation which is difficult to make sense of, the many worlds interpretation which is equally crazy and leading to all sorts of absurdities, and some sort of hidden variable theory. You didn’t go into too much about why the hidden variable idea does not work. Anyway the discussion raised some interesting issues about the goals of scientific theories and what they can hope to tell us about the world, so I think Quantum mechanics leads to some interesting philosophy. I thought you guys did a good job of explaining many of the issues involved in a clear and concise manner. I thought the guest speaker also did very well. The discussion of Nagel also brought up the distinction between science and pseudoscience. I think it is good to have some level of skepticism in most things–including science. But I think a scientific philosophy is the best way at this time to proceed with trying to understand the world.

  2. Avatar of Wes Alwan

    Wes Alwan

    January 21, 2010

    Thanks Dan, glad you enjoyed it. I’m actually sympathetic to both of the positions you describe — the hidden variable theory and the cogsci/Kantian position you describe. I know our guest Dylan discussed the former, but I’m not sure where he came out on that — I forget if it’s been discarded based on the evidence. As for the Kantian position (which I hesitate to mention, since Mark accused me of having a blowup doll of Kant in my room), I think it’s plausible to suggest that quantum inconsistencies belong to a “phenomenal” level but go away at the level of some underlying reality. On the other hand, then we’re confronted with the many problems involved with this kind of epistemology!

  3. Dan B

    January 23, 2010

    I agree. It might bring in doubt in the validity of scientific observations if we did something like that. There are also a few different versions of the many-worlds interpretation, including the branching version you guys talked about. I have heard of other ones that describe the universe like a sheet of paper in a book. Of course most people find the many-worlds pill a difficult one to swallow. When I first heard of it I thought it was probably a waste of time to even consider. However, it does have some strengths. These aren’t all of them but for one it avoids the need for observers to collapse the world into a particular state, which seems odd. That seems to put us too close to the center of the universe again. Many-worlds also seems to address the problem of this world existing and seeming to be perfectly suited for us. I believe in evolution, but in order for evolution to take place–-for the atoms and molecules to form–the universe has to be set up just right. From what I understand, physicists tell us here are many other ways this universe could have been. If one of the constants in physics were even a tiny fraction different, life could not exist in this universe. If the universe could so easily have been different, and there is only one universe, why did it happen to be one that was suited for life? I am sure you have heard of an argument like this from theists. It seems very improbable that we would get so lucky if there were only one shot at it. However, if every possible universe existed, or at least an extremely high number of universes existed, then this universe may not be as improbable. Then there would be lots of universes like this and we would not be so amazing after all. I am not saying I buy it, but there is something to be said for it. It is a good one to put away in your toolbox. It defeats the need for a creator.

    • Nullifidian

      June 1, 2011

      I’m quite late to this discussion, but if I’m going back through and listening to previous podcasts, then so could anybody else.

      The notion that the universe has a few tightly constrained constants that, without being at or near their present levels, would have made it impossible for life to exist is a common theistic argument, and there are more ways of combating it than merely by arguing for multiverses (which albeit is a reasonable outcome of inflationary theory).

      One of the flaws in the argument is that they only consider the effect of changing one constant at a time. Victor Stenger, a retired physicist, has argued that we should see what would happen if *all* the fundamental constants were equally likely to vary (because that’s how it would play out in any “birth” of the universe scenario), and he has shown through computer modeling that this considerably improves the likelihood of having a “life-friendly” universe.


      Also, there is a probabilistic argument that can be used against the “anthropic principle”—namely, that the people who propose it get their probabilities backwards. Michael Ikeda and Bill Jeffreys have written an article for a proposed FAQ on the “anthropic principle” using the analogy of a poker hand. You are unlikely to win a hand of poker because you are dealt a royal flush; most hands of poker are won with less. But it would be absurd to say that, having been dealt a royal flush, you would be almost certain to *not* win the hand. Likewise, if it is unlikely that a naturalistic universe would be life-friendly, it does not follow from this fact alone that, having a life-friendly universe, it is unlikely to be a naturalistic one. In fact, the overwhelming probability as far as we can determine is that the universe would be naturalistic. If life required a constant adjustment out of nowhere so that it could continue to exist, if everything were dead set against life existing and it did anyway, then we would be entitled to infer that the existence of life was a miracle. But if life needs no support beyond that already provided by the universe, then where is the warrant for inferring a God? It looks very much like having one’s cake and eating it too.

  4. Dan B

    January 23, 2010

    My observations lead me to believe that my observations are not reliable.

    This sentence is false.

    • faceglider

      April 11, 2013

      Which sentence?

  5. Nullifidian

    June 1, 2011

    Also, one other recommendation. Victor Stenger, whom I mentioned above, has often dealt with philosophical issues in his books. Three of his best are The Unconscious Quantum: Metaphysics in Modern Physics, where he details the interpretative contexts one can bring to QM of which the Copenhagen Interpretation is just one of many; Timeless Reality, where he argues that much of quantum weirdness becomes straightforward by simply rejecting the notion of an arrow of time operating at the quantum level; and The Comprehensible Cosmos.

  6. Aaron

    February 13, 2012

    Ahoy! Recently started listening to PEL. After listening to a few recent episodes, I started from the beginning. Started listening to Episode 13 today. I was pretty disappointed by the discussion of the Nagel paper. I’ve only skimmed through Nagel’s paper, but I can’t say I was terribly impressed, and that I agree with much of Leiter says.

    I think Negel has jumped into this fight mistream- he appears to be ignorant of much of its history and misunderstands a lot of the objections to ID. I think a lot of people, including Dawkins, would agree that ID can be evaluated as a scientific question. Indeed, in some of Dawkins popular science books he talks about some of the evidence we might expect given the claims made by advocacy groups such as the Discovery Institute. These are empirically testable, and have so far failed these tests.

    When it comes to determining policy on public education, ideas which have not met a reasonable burden of proof should be left out of the classroom. With very few exceptions, this is what Richard Dawkins, Eugenie Scott, etc want- if they make no testable hypotheses or their hypotheses continually fail their tests, then it doesn’t belong in a public school being taught as a science on par with modern evolutionary theory and everything else we teach in our science classrooms.

    Wes stated that “there are all sorts of unresolved problems with evolution.” Of course there are- that’s true of science as a whole. And philosophy, for that matter. If there are “unresolved problems” in philosophy, should we dedicate equal time in every philosophy class discussing what the world’s revealed religions teach? A class on Nietzsche? Half Nietzsche and half Bible. The analogous situation for biology classes is what the ID proponents are after, and what the Dover trial rejected.

    I should disclose my bias- I’ve an amateurish interest in philosophy at best and (most of) an undergraduate degree in biology along with a handful of grad-level electives.

  7. Adam

    February 13, 2012

    Great episode.

    As a side note, my newb understanding of Copenhagen interpretation is that it has some inherent anthropocentrism and that the Many worlds interpretation is starting to be preferred way to think about qm because it doesn’t require people to play a role.

    Perhaps something to talk about in a future episode.

    • Avatar of Dylan Casey

      Dylan Casey

      February 13, 2012

      Glad you liked the episode, Adam.

      The Copenhagen interpretation has certainly quite a few variants and, indeed, some interpret things like the “measurement problem” in a very anthropocentric way, making it a problem of “choosing” what to measure. I think that it’s pretty clear that such an interpretation is a mistake, after all, QM behavior happens all the time everywhere in the universe. Additionally, the “collapse” of the probabilities (i.e., the actualizing of the possibilities, to use Heisenberg’s Aristotelian-inspired language) doesn’t require a human/consciousness interaction. In the end, the wavefunction collapse becomes a process of constraining the possibilities.

      As far as many worlds goes, admittedly, I haven’t studied it deeply. I’m primarily put-off from it simply because it smacks of over-enthusiastic mathematization; an equating of mathematical possibility with physical reality. QM mechanical interactions (as said) occur all the time everywhere, with physical interaction depending upon (something like) wave collapse all the time, not just when nosey physicists setup detectors and peak. So, prima facie, the sheer (ongoing) multiplicity of the worlds since the beginning of time seems patently absurd. It may be that I’m over-simplifying the view and that there is a straightforward answer to this absurd multiplicity that ends with a simple multiplicity of mutually interacting worlds. Still, such a reality ought to have measurable effects.

      • Ryan

        February 13, 2012

        Both interpretations share in the fundamental absurdity that there should be an equal functional role for a macroscopic ensemble of biophysiological processes that make up a quantum physicist, and also interactions between individual subatomic particles, to play in bringing about state collapse. What I think the MWI has in its advantage is that it turns the multiplicity of state collapse in to a presupposition rather than leaving it as a weird conclusion landed upon based on theoretically informed observations that these completely distinct kinds of entities just so coincidentally share similar forms of causation. “Heisenberg’s Aristotelian-inspired language” is exactly what is wrong with CI. While there are no empirical differences so far to contrast their accuracy that I am aware of, which makes it curious that either should become so much more successful than the other, the many-worlds proponent posits that his model of the world is at least partially true despite our ongoing amendments to it, or as you colorfully put it, “QM behavior happens all the time everywhere in the universe.” The copenhagen adherent thinks the model he is building of the world is only a joke and holds no connection with reality whatsoever. The implications for life itself becoming more completely mathemetized through the fields of physics, biology, sociology, haven’t been coherent for centuries now, and yet they not only persist but become more prevalent by the day. In fact the MWI exactly matches the way we make statistical predictions about the future, and also how we formulate counterfactual claims about the past.

      • Adam

        February 23, 2012

        Great point about QM going on all the time, everywhere.

        Maybe its not the MWI interpretation that is absurd.

        Maybe its that our intuition of the world that is absurd, when applied to the way quantum physics tells us the world works at a quantum scale.

  8. Joan

    March 31, 2012

    great episode.
    attending a lecture/discussion tmrw on einstein and the philosophy of science, and this was great prep.
    around 1:40, one of you guys mention crohn’s disease. maybe seth? just wanted to say: keep strong. 30 years dx for me, 10 for my mom, 5 for my daughter.
    science is what gives me hope for a cure.
    got keep the hope.

  9. Brendan

    November 11, 2012

    I find it interesting how people often resist what quantum mechanics suggests about realism. Whether or not things really exist beyond mere observations, it seems perfectly intuitive that things which aren’t observed should be uncertain. Why be surprised when someone tells you that without measurement all they can talk about is probabilities? I am perfectly satisfied with thinking that things which are unobserved have their reality become fuzzy, just as their states become uncertain.

    People also seem perplexed because this seems to put humans in a special place in the universe, since it is apparently our observations that count. The slits themselves certainly don’t count as observers in a double slit experiment. For me, it seems perfectly obvious why humans seem special: it is merely because I am human and the entire concept of an observer is subjective. Other humans count as observers only when they share what they observed with me, allowing me to observe what they observed. Until they do that, the results of the experiment are unobserved and only exist as a wave function of possible results. As soon as they talk to me about it, the wave function collapses and I hear about some specific results as if they had always been specific.

    The whole universe gets along perfectly fine without any observers at all, assuming because of realism that it even exists. Objects on the other side of the galaxy can exist only as wave functions and interact freely without ever collapsing the wave functions. The principle of a quantum computer, for example, is that particles can interact in a superposition of states and thereby perform multiple computations simultaneously. That couldn’t work if just interacting caused wave functions to collapse.

    So there’s nothing strange about giving humans special status as observers, since being an observer is subjective and the universe gets along fine in places without observers.

    It’s also not strange to suppose that there are many worlds and that branching happens with every observation. I was blown away when it was said that the enormous number of worlds implied by that interpretation was absurd, apparently just because it was large. According to science large numbers are everywhere, from the size of the universe to the particles that make up ordinary objects. It seems obvious to me that no number, no matter how large or small, could suggest that something metaphysical was absurd.

    The part that surprises me is that the physical properties of the world change depending on whether it is being observed or not. You can never really be sure of what might be happening behind the back of your head, but it is unsettling to have science confirm that the world behind you is an alien and magical place that is foreign to the world before your eyes, even only slightly.

    The first idea that occurred to me after listening to this podcast was to wonder if it might be possible to perform a double-slit experiment with larger particles, such as tennis balls. I have no doubt that unobserved large objects exist as wave functions just as small objects do. The difficulty with large objects is that they are so easy to observe. You would need a sealed room and a mechanical ball server, all setup to make it impossible to ever investigate exactly how the tennis balls flew. A human server wouldn’t work, not because the human would be observing the path of the tennis balls, but because you couldn’t keep the human server sealed in the room forever and some trouble-maker would be bound to ask the server about it later and that would be an observation. I expect the best chance would be do set up the room so that immediately after the experiment it is obliterated in some sort of incinerator, thus eliminating all chance of observation.

    So you get a perfectly sealed room that is immune to all investigation now and in the future and inside you do a double-slit experiment with tennis balls passing through two slits and hitting a wall. Make the wall instrumented to detect the position of any tennis balls that bounce off it, then it would not surprise me at all to find that the tennis balls behaved like waves according to the detector. But that would only happen if we were in a world where no one ever discovered any evidence about the actual flight of the balls. As soon as that happens, the wave function would collapse with us ending up in a world where the tennis balls flew as particles.

    I would love to see an experiment like that. If it managed to turn tennis balls in to waves, the metaphysical implications would be profound, though not really surprising.

    • Guy

      August 29, 2013

      To me this corresponds with Deleuze’s transcendental empiricism. To my own very limited understanding, a lot of his writing deals with this type of subject-object relationship, through his constructions of virtual/actual relationships, concept-affect etc. As Badiou claims, the terms shift but the metaphysics stays the same. Its interesting in his last book when he, with Guattari, adds functives to the mix; relative functions which seek to predict, or near predict, the virtual/actual interaction.

  10. Avatar of Dylan Casey

    Dylan Casey

    November 12, 2012

    The tennis ball experiment won’t work — the wavelength is just too short. However, it would be interesting to investigate large-scale quantum phenomena with long-wavelength photons. Microwaves have wavelengths of tens of centimeters. Radio waves, be them AM radio or waves emitted from distant galaxies have wavelengths the size of football fields. It would be very interesting to examine single photon interference from such waves. The genuine difficulty in doing so is that the energies are very, very low for such wavelengths — the longer the wavelength, the smaller the energy. Detecting very small energy depositions is notoriously difficult.

    • Brendan

      November 14, 2012

      I see what is meant by the wavelength being too short. Studying the topic a little bit more has lead me to de Broglie’s matter waves and how the wavelength of matter is based entirely on its kinetic energy.

      Intuitively I expected that the wave function of an isolated tennis ball and serving machine would have spread from the furthest left that the ball might fly to the furthest right that the ball might fly, and over time based on how likely the ball is to fly at each moment. Based on my limited understanding of the situation, I expected that those distances would lead to the wavelength of the wave function of the tennis ball because this is clearly how the uncertainty about the position of the ball spreads out, but merely being uncertain about something doesn’t always need to lead to physical consequences like interference patterns.

      Until you open an envelope and read the letter inside, the letter exists in a superposition of all possible letters that the envelope could contain, but no more than that. Being ignorant only creates serious additional possibilities for very small or very fast things. Ordinary objects are stuck with being only what ordinary objects can be according to traditional mechanics.

      To get actual interference patterns from tennis balls, it seems that the only way is accelerating the tennis balls to totally impractical velocities. I thought that perhaps a large isolated system would cause uncertainty to compound upon uncertainty and we might find that it behaves in a large-scale unrealistic manner. Personally I hold no realism for a totally isolated system: if no observation ever occurs inside a box, then science has nothing to say about the content of the box and no rules apply, and I expect that to be true to various degrees in a range from no observation to maximum observation. It seems that in the case where we know the content of the box because we built the box and put in the mechanical server with tennis balls, and we make observations about where the tennis balls bounce off the walls, then we can expect to find the tennis balls flying as tennis balls always do thanks to all those observations and their effects on the wave function of the system.

      The world is a little bit less magical than it could have been, which is probably a good thing.

  11. Avatar of Wayne Schroeder

    Wayne Schroeder

    November 12, 2012

    Amazing. Brendan is giving us the perception of “reality” from the position of the object, rather than the subject. I have never seen such total identification by a subject with an object, such that it would make sense to approach the double slit experiment with tennis balls.

    Brendan’s point appears especially not to be about objectivity (quantum physics) as experienced by a subject, but objectivity as experienced as a logical object. Thus, whether about realistic objects or about “impossible” objects (i.e. tennis balls as photons, etc.), truth is about objects only.

    I would add that Brendan’s position is about ultimate reductionism.

  12. Tony Gilkerson

    January 6, 2013

    I liked your guest. I am only up to #13 so I hope I find that he rejoined you guys. Great topic, I am looking forward to your other philosophy of science episodes. Also, I like the variety in the songs at the end. I usually skip over gratuitous songs at the end of podcasts but after hours of good discussion I am finding that I need the song to wind down. Kind of like how they always meditate at the end of yoga sessions. In all seriousness great job on the philosophy and music.

    • Avatar of Seth Paskin

      Seth Paskin

      January 7, 2013

      Thanks Tony. You’ll be happy to know that the guest – Dylan Casey – joined us a permanent member.

  13. Joe

    August 21, 2013

    Great podcast fellas. Got a more nuanced understanding on this topic, so thank you.

    One point, which you guys kept returning to, but never seemed to resolve, which has eluded me as well, is the question of whether this uncertainty inherent in quantum observations is by virtue of measurement limitations or a fact of the universe itself.

    Is there no difference in these propositions, is one inquiring for an ontological understanding and the other from the technological pragmatics of scientific investigation and thus are two different questions entirely, or is a consequence of the theory that the properties of any quantum state simply cannot be known with mutual precision?

    Whatever the case, what I would really like is the chain of reasoning for why QM’s CI is taken as a “complete theory” and that uncertainty is fundamental.


    • Avatar of Dylan Casey

      Dylan Casey

      August 22, 2013

      The uncertainty in quantum mechanics inherent in QM itself — Heisenberg’s Indeterminacy Principle, in which the determination of the position and the momentum of a particle have a joint, minimum, value is the embodiment of that character of QM. That indeterminacy extends to other conjugate variables, energy and time, for example.

      It is a separate question regarding how we take that character of QM. Heisenberg, in the book we read for this podcast and other places, articulates the indeterminacy principle in terms of the impossibility of making a measurement without disturbing the original system. For very small systems, the act of measuring then makes a huge effect on the system itself. Alternatively, one might take the success of QM to be an indication that its underlying principles are, in fact, true about the character of the universe itself, in which case, one is lead to saying that the uncertainty is “fundamental”. (We then get into a discussion about exactly what we mean by fundamental, but I won’t digress.) Yet another alternative would be to take that fundamental uncertainty and run with it, both in terms of physical theory (many worlds theory) and in terms of analogies with human thought, ethics, morality, etc. Yet again, one might (as Einstein did) consider that fundamental uncertainty a fundamental flaw in the theory itself and a clear sign that it must eventually be replaced.

      Indeed, the question of how to take the implication of the uncertainty inherent in QM is at the root of almost every dispute about interpreting it.

      The reason for considering QM a “complete theory” (which it isn’t, unless you consider quantum field theory a little extra hair on the legs of quantum mechanics) is that the measurements of physical parameters predicted by quantum mechanics (and quantum field theory) agree with one another to absurd precision — out as far as we can measure. And, every time we look closer, it just continues to agree. That, in the end, is the reason why one might take QM seriously and try to understand it’s implicit worldview. Such an discussion has been raging since QM was thought up.


  14. Avatar of Wayne Schroeder

    Wayne Schroeder

    August 22, 2013


    Let’s say that we agree with Steven Hawking of the Grand Design that because we can explain the origin and nature of the universe using the Theory of Everything (based on quantum mechanics, string theory, etc.), that both God and Man, (man in the sense of philosophy), are now outmoded. The explanation by science/physics of the nature of reality eliminates both God or Man as having explanatory power for the nature of reality.

    The question here is what is reality and how do we explain it?

    Is nature something that is separate from human nature, from the nature of Man?

    Does there need to be a one-to-one correspondence between nature/substance and the nature of man?

    Why and who says?

    Can the uncertainty of human nature be fundamental regardless of the uncertainty of nature, separate from a one-to-one correspondence?

    Can the reality of the nature of Man supersede the reality of nature, merely and especially because we are Man?

    If not, why not? And can uncertainty still be fundamental?

  15. David Newberry

    August 27, 2013

    If you ever do another episode on QM, I hope you devote some more time to the “pilot-wave” theory (originated by de Broglie, and picked up years later by Bohm). As was briefly alluded to in this episode, Bohm helped clarify the status of quantum mechanics and non-local. See, e.g. http://plato.stanford.edu/entries/qm-bohm/

  16. Matthew Brower

    February 27, 2014

    Pertaining to your reservations regarding the need for an observer to explain the uncertainty principle prior to human beings, all biological life possesses the ability to react to the world around it, as well as atoms, all of these are forms of awareness. Sure, our awareness is that of the highest degree, we have a brain capable of rendering massive amounts of this static into a followable reality. But, a bacterial cell too has the ability to perceive, albeit a limited version of consciousness, limited to the beings physical apparatus to render that static. In this way, we can see that the primal atoms from the beginning, as well as the refined perceptions of the human, are one thing, ultimately building the experience of reality we see. Our consciousness, it could be, is an accumulative effect of all these processes culminating or incarnating into a super being, aka the human. Just my two cents boys, bravo for the podcast. I stumbled upon it after googling “quantum implications on philosophy”.

  17. Dasein

    March 15, 2014

    Sorry for coming to the party late.

    An author and a book was mentioned in the podcast as a better read than the author used. It was described as a philosopher who had a great grip on the physics and was a great read. Who was that?

    Thank you in advance.

    • Avatar of Dylan Casey

      Dylan Casey

      March 15, 2014

      Sorry, but I don’t remember at all which book was mentioned in particular. The only that comes to mind in “The Philosophy of Quantum Mechanics: The Interpretations of Quantum Mechanics in Historical Perspective” by Max Jammer.

      • Dasein

        March 18, 2014


        Just as a question (I suppose for Dylan), the newspapers today (!) are talking of the discovery of gravitational waves as paving the way for a unifying theory of everything. I don’t get it. The logic seems odd. Surely, precisely because it follows earlier predictions of said waves it opens up no avenues for coming up with a new theory….?

        (by the way, Dylan is extraordinary as a guest on this podcast)

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